// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This file implements binary search.

// package sort -- go2cs converted at 2022 March 13 05:27:34 UTC
// import "sort" ==> using sort = go.sort_package
// Original source: C:\Program Files\Go\src\sort\search.go
namespace go;

using System;
public static partial class sort_package {

// Search uses binary search to find and return the smallest index i
// in [0, n) at which f(i) is true, assuming that on the range [0, n),
// f(i) == true implies f(i+1) == true. That is, Search requires that
// f is false for some (possibly empty) prefix of the input range [0, n)
// and then true for the (possibly empty) remainder; Search returns
// the first true index. If there is no such index, Search returns n.
// (Note that the "not found" return value is not -1 as in, for instance,
// strings.Index.)
// Search calls f(i) only for i in the range [0, n).
//
// A common use of Search is to find the index i for a value x in
// a sorted, indexable data structure such as an array or slice.
// In this case, the argument f, typically a closure, captures the value
// to be searched for, and how the data structure is indexed and
// ordered.
//
// For instance, given a slice data sorted in ascending order,
// the call Search(len(data), func(i int) bool { return data[i] >= 23 })
// returns the smallest index i such that data[i] >= 23. If the caller
// wants to find whether 23 is in the slice, it must test data[i] == 23
// separately.
//
// Searching data sorted in descending order would use the <=
// operator instead of the >= operator.
//
// To complete the example above, the following code tries to find the value
// x in an integer slice data sorted in ascending order:
//
//    x := 23
//    i := sort.Search(len(data), func(i int) bool { return data[i] >= x })
//    if i < len(data) && data[i] == x {
//        // x is present at data[i]
//    } else {
//        // x is not present in data,
//        // but i is the index where it would be inserted.
//    }
//
// As a more whimsical example, this program guesses your number:
//
//    func GuessingGame() {
//        var s string
//        fmt.Printf("Pick an integer from 0 to 100.\n")
//        answer := sort.Search(100, func(i int) bool {
//            fmt.Printf("Is your number <= %d? ", i)
//            fmt.Scanf("%s", &s)
//            return s != "" && s[0] == 'y'
//        })
//        fmt.Printf("Your number is %d.\n", answer)
//    }
//
public static nint Search(nint n, Func<nint, bool> f) { 
    // Define f(-1) == false and f(n) == true.
    // Invariant: f(i-1) == false, f(j) == true.
    nint i = 0;
    var j = n;
    while (i < j) {
        var h = int(uint(i + j) >> 1); // avoid overflow when computing h
        // i ≤ h < j
        if (!f(h)) {
            i = h + 1; // preserves f(i-1) == false
        }
        else
 {
            j = h; // preserves f(j) == true
        }
    } 
    // i == j, f(i-1) == false, and f(j) (= f(i)) == true  =>  answer is i.
    return i;
}

// Convenience wrappers for common cases.

// SearchInts searches for x in a sorted slice of ints and returns the index
// as specified by Search. The return value is the index to insert x if x is
// not present (it could be len(a)).
// The slice must be sorted in ascending order.
//
public static nint SearchInts(slice<nint> a, nint x) {
    return Search(len(a), i => a[i] >= x);
}

// SearchFloat64s searches for x in a sorted slice of float64s and returns the index
// as specified by Search. The return value is the index to insert x if x is not
// present (it could be len(a)).
// The slice must be sorted in ascending order.
//
public static nint SearchFloat64s(slice<double> a, double x) {
    return Search(len(a), i => a[i] >= x);
}

// SearchStrings searches for x in a sorted slice of strings and returns the index
// as specified by Search. The return value is the index to insert x if x is not
// present (it could be len(a)).
// The slice must be sorted in ascending order.
//
public static nint SearchStrings(slice<@string> a, @string x) {
    return Search(len(a), i => a[i] >= x);
}

// Search returns the result of applying SearchInts to the receiver and x.
public static nint Search(this IntSlice p, nint x) {
    return SearchInts(p, x);
}

// Search returns the result of applying SearchFloat64s to the receiver and x.
public static nint Search(this Float64Slice p, double x) {
    return SearchFloat64s(p, x);
}

// Search returns the result of applying SearchStrings to the receiver and x.
public static nint Search(this StringSlice p, @string x) {
    return SearchStrings(p, x);
}

} // end sort_package
